1. Field of the Invention
The present disclosure relates to an organic light emitting diode (OLED) display device. More particularly, the disclosure relates to an OLED display device having an improved image quality.
2. Discussion of the Related Art
An OLED display device of flat panel display devices, which may be referred to as an organic electroluminescent display device, has high brightness and low driving voltage. In addition, because it is self-luminous, the OLED display device has an excellent contrast ratio and a ultra thin thickness. The OLED display device has a response time of several micro seconds, and there are advantages in displaying moving images. Also, the OLED display device has wide viewing angles and is stable under low temperatures. Since the OLED display device is driven by low voltage of direct current (DC) 5V to 15V, it is easy to design and manufacture driving circuits.
Accordingly, the OLED display device has been widely used as a television, a monitor, a mobile phone, and so on.
Hereinafter, a structure of an OLED display device will be described in more detail.
FIG. 1 is a cross-sectional view schematically illustrating a pixel region of an organic light emitting diode (OLED) display device according to the related art.
The OLED display device 1 according to the related art includes an OLED substrate 10 having an array element and an organic light emitting diode E and an opposite substrate 70 for encapsulation.
The array element on the OLED substrate 10 includes a switching thin film transistor (not shown) connected to gate and data lines and a driving thin film transistor DTr connected to the organic light emitting diode E. The organic light emitting diode E includes a first electrode 47 connected to the driving thin film transistor DTr, an organic light emitting layer 55 and a second electrode 58.
Light emitted from the organic light emitting layer 55 is output through the first electrode 47 or the second electrode 58, and thus the OELD display device 1 displays an image.
At this time, the first electrode 47 is formed of a transparent conductive material having a relatively high work function such as indium tin oxide (ITO) and functions as an anode electrode. The second electrode 58 is formed of a metallic material having a relatively low work function and functions as a cathode electrode.
The related art OLED display device has a pixel consisting of three sub-pixels of red, green and blue.
To improve color display quality and brightness, an OLED display device having a pixel consisting of four sub-pixels of red, green, blue and white has been suggested.
FIG. 2 is a cross-sectional view illustrating part of an OLED display device having a pixel consisting of four sub-pixels according to the related art. FIG. 2 omits an opposite substrate and an array element and schematically shows a color filter layer 45 and an organic light emitting diode E. For convenience of explanation, white, red, green and blue sub-pixels of the pixel are defined as first, second, third and fourth sub-pixels SP1, SP2, SP3 and SP4.
In FIG. 2, the pixel P of the OLED display device 2 consists of four sub-pixels SP1, SP2, SP3 and SP4. A first electrode 47 as an anode electrode is formed at each of the sub-pixels SP1, SP2, SP3 and SP4. An organic light emitting layer 55 emitting white light is formed on the first electrode 47 all over a display area. A second electrode 58 as a cathode electrode is formed on the organic light emitting layer 55 all over the display area.
Here, red, green and blue color filter patterns 45a, 45b and 45c are formed under the first electrodes 47 in the second, third and fourth sub-pixels SP2, SP3 and SP4, respectively. At this time, although not shown in the figure, a black matrix is formed in boundaries of the color filter patterns 45a, 45b and 45c. 
In the OLED display device 2, the organic light emitting layer 55 emits white light, and the white light passes through the red, green and blue color filter patterns 45a, 45b and 45c in the second, third and fourth sub-pixels SP2, SP3 and SP4 to produce red, green and blue light. Since a color filter layer is not formed in the first sub pixel SP1, the white light passes through the first sub pixel SP1 as it is, thereby expressing white.
At this time, to improve color purity and brightness of the red, green and blue light from the second, third and fourth sub-pixels SP2, SP3 and SP4, the first electrodes 47 of the second, third and fourth sub-pixels SP2, SP3 and SP4 have different thicknesses to cause a micro cavity effect.
For the micro cavity effect, the first electrode 47 has a double-layered structure of a lower layer 47a and a second layer 47b. The lower layer 47a is formed of a metallic material having relatively high reflectivity and a transmissive property at a relatively thin thickness such that selective reflection is made. The metallic material may be silver (Ag), for example. The upper layer 47b is formed of a transparent conductive material having relatively high work function. The transparent conductive material may be indium tin oxide (ITO), for example.
The micro cavity effect is a phenomenon that, by making a difference in thicknesses of material layers through which light passes, that is, optical distances, light emitted from the organic light emitting layer 55 is repeatedly selectively reflected between specific layers and light with changed spectrum and increased optical intensity is finally transmitted through the first electrode 47 or the second electrode 58.
Like this, by making a difference in the thicknesses of the first electrodes 47 at the sub-pixels SP2, SP3 and SP4, the OLED display device 2 may have the improved brightness and color purity because of the micro cavity effect.
At this time, if the first electrode 47 of the first sub pixel SP1 expressing white includes a lower layer of silver (Ag) with an upper layer of the transparent conductive material, the first sub pixel SP1 emits light with a specific wavelength range, and color coordinates fall through. Therefore, the first electrode 47 of the first sub pixel SP1 has a single-layered structure of indium tin oxide without the lower layer of silver (Ag).
Meanwhile, as shown in FIG. 3, which is a graph showing brightness of a related art OLED display device having the above-mentioned structure according to viewing angles, the brightness in the second, third and fourth sub-pixels of the related art OLED display device expressing red, green and blue is remarkably decreased according to the viewing angles due to the difference in the thicknesses of the first electrodes. On the other hand, the brightness in the first sub pixel of the related art OLED display device expressing white is slightly decreased according to the viewing angles because of the first electrode without a lower layer.
Accordingly, the OLED display device having the above-mentioned structure has low image qualities due to the difference in decreases of the brightness of the white sub pixel and the red, green and blue sub-pixels.